1. Field of the Invention
This invention relates to a method and an apparatus of controlling the injection of a die casting machine and, more particularly, it also relates to a method and an apparatus of controlling the injection of a die casting machine so as to shift the injection speed from low to high in the course of an injection stroke.
2. Prior Art
In a conventional die casting machine for injecting molten metal fed to an injection sleeve into a cavity of a metal mold by means of an injection plunger to charge the metal mold with molten metal, the plunger rod is advanced at a low rate in the initial phase of a charging stroke until the front end of a charge of molten metal gets to the gate of the metal mold and the frontal space of the plunger rod is filled with molten metal in order to prevent the charge of molten metal from containing air bubbles and, once the frontal space of the plunger rod is filled with molten metal, the advancement of the plunger rod is shifted to a high rate in order to fill the cavity of the metal mold at an enhanced speed.
The injection speed of a die casting machine is determined as a function of the wall thickness of the product and the time required for the product to become solidified and the operation of injecting molten metal and filling the cavity of the metal mold with molten metal has to be completed before the charge of molten metal is solidified. A higher injection speed is required as products having a reduced wall thickness and a complicated profile are produced in recent years.
The kinetic energy Q of a charge of molten metal when it is injected into a metal mold cavity is defined Q=m.multidot.V.sup.2 /2, where m is the mass of the charge and V is the injection speed (the rate of advancement of the plunger rod). Thus, the kinetic energy increases as a higher injection speed V is used as we see in recent years so that the plugger rod is required to reduce its speed accurately Just before the completion of the charging stroke.
The timing for starting the speed reduction is simply determined by the position of the plunger rod if the charge of molten metal brought into the injection sleeve from a ladle for each die casting cycle is always constant. However, in reality, the charge of molten metal brought into the injection sleeve from a ladle shows fluctuations from cycle to cycle.
If the timing for starting the speed reduction is predetermined corresponding to a defined charge of molten metal and hence to a given position of the plunger rod, the plunger rod prematurely starts reducing its speed if the current charge is short of the defined charge. The net result will be a pressure rise time lag due to an insufficient charge and an interrupted flow of molten metal. On the other hand, the plunger rod starts reducing its speed after passing an optimal time point to bring forth flashes or fins on the product as a result of overcharging if the current charge exceeds the defined charge.
In an attempt to solve these problems, there has been proposed an apparatus for controlling the injection speed. The apparatus comprises a metal charge front sensor disposed at the gate of the metal mold for exactly detecting the time when the front end of a charge of molten metal arrives the gate. During an injection stroke at high injection speed, an injection control program is executed by using a speed profile which is predetermined according to a position of a plunger rod and which includes a forced speed reduction part before the completion of injection. The injection speed control program variably starts operating as a function of the time when the metal charge front sensor detects the front end of the charge.
However, since the metal charge front sensor of a proposed apparatus for controlling the injection speed has to be arranged in the metal mold, it cannot be used with an existing metal mold without modifying it. Additionally, the metal charge front sensor is often accompanied by a number of problems including malfunction and broken wires to make the maintenance of itself and the metal mold rather cumbersome.